1. Field of the Invention
The present invention relates to an image recording apparatus.
2. Description of the Related Art
An ink jet recording apparatus which performs printing (recording) by discharging ink onto a recording medium is known as one of the conventional image recording apparatuses.
Ink jet recording apparatus have been rapidly spread recently because they are non-impact type recording apparatus and producing little noise, and color image recording can be easily implemented by using multiple inks.
FIG. 1 is a schematic perspective view showing a conventional ink jet recording apparatus.
In FIG. 1, a recording medium 5 wound in a roll passes transport rollers 1 and 2, is held by paper incrementing rollers 3, and is incremented in the direction of f in FIG. 1 by the driving of a subscanning motor 4 coupled to one of the incrementing roller 3. Across the recording medium 5, guide rails 6 and 7 are disposed in parallel, and a carriage 8 is mounted thereon. The carriage 8 moves in the lateral direction so that a recording head unit 9 mounted thereon is scanned in that direction. On the carriage 8, four color recording heads 9Y, 9M, 9C and 9Bk for yellow, magenta, cyan and black are mounted, and four ink reservoirs corresponding to those colors are disposed. Each head includes a orifice array including a plurality of ink orifices, and orifice arrays of the heads are disposed in parallel. The recording medium 5 is intermittently incremented by the amount of recording of the head unit 9, and the head unit 9 is scanned in the direction P discharging ink droplets in accordance with a binary image signal while the recording medium is stationary.
With such an ink jet recording apparatus, the recording medium characteristics are very important. In particular, ink running characteristics on a recording medium have strong influence on image quality. As one of the indices representing the ink running characteristics of the recording medium, a "spreading rate" is known. It indicates how many times an ink droplet ejected from an ink jet nozzle will spread after arriving on a recording medium, and is given by the following equation: EQU spreading rate=dot diameter on the recording medium/ink droplet diameter ejected from the nozzle
For example, if an ink droplet of 30 .mu.m diameter during traveling forms a dot of 90 .mu.m diameter on the recording medium, the spreading rate of the recording medium is 3.0. With recording media of small spreading rates, the image intensity is low, and hence, it is difficult to achieve smooth texture, high quality images.
On the other hand, although recording media of a high spreading ratio can increase the image intensity, the following problems arise.
A serial scanning type ink jet recording apparatus as shown in FIG. 1 sequentially records images of width d as indicated by (1), (2) and (3) in FIG. 2 by scanning in the direction A the recording head unit 9 including a plurality of ink discharge orifice arrays disposed in parallel. For example, when the number of orifices per array is 256, and the recording density is 400 dots/inch (dpi), the recording width d becomes 16.256 mm (=265.times.25.4/400).
When the volume of ink landing on the recording medium is small, the actual width of the recorded image is approximately equal to the recording width d because the ink is sufficiently absorbed by the recording medium. Accordingly, when recording is performed by repeating the scanning of the recording head unit 9 in the direction A and the subscanning of the head unit 9 by the amount of d in the direction B, joint portions between two adjacent images recorded by two continuous scannings present little problem as shown in FIG. 2.
The image width, however, may increase to d+.DELTA.d owing to ink running when recording high density portions where the volume of ink is great because the recording medium of a large spreading ratio cannot sufficiently absorb ink. In such a case, the subscanning of the head unit 9 by an amount of d will cause an overlap of images by the amount of .DELTA.d as shown in FIG. 2B, thereby resulting in black lines at the overlapped portions. In contrast with this, if the subscanning of the head unit 9 is set as the amount of d+.DELTA.d, white lines will appear at low density portions where the volume of ink is small.
The extra amount .DELTA.d of the image width at high density portions changes depending on the spreading rate of a recording medium, or the volume of ink arriving at the recording medium, and increases consistently with the spreading rate and the ink volume. For this reason, to prevent black lines from occurring, it is necessary to use a recording medium of a small spreading rate, or to reduce the ink volume. In this case, however, the image intensity decreases as stated before, which presents a problem in that smooth texture, high quality images cannot be obtained.
To solve such a problem, the applicants of the present invention proposed the U.S. Ser. No. 679,147. In this proposal, if values of an image signal at the boundaries of the serial scanning exceed a predetermined amount, they are reduced before being applied to recording elements involved in recording the boundaries. This makes it possible to reduce the ink volume discharged from nozzles associated with the recording elements, thereby preventing black lines from occurring at high density, boundary portions.
In this method, the image signal applied to the recording elements associated with the nozzles at the edges of the discharge orifice arrays is converted by a table, the function of which is shown in FIG. 3: if the input image signal to the table exceeds a predetermined value T, the output of the table fed to the recording elements is restricted at a fixed value F. Thus, in the case where the density of the image signal applied to the recording elements associated with edge nozzles is rather high, the volume of ink discharged from the edge nozzles is restricted, thereby circumventing the occurrence of black lines at the joint portions.
On the other hand, a recording apparatus has been proposed which includes a plurality of recording heads discharging inks of different concentration to improve the gradation of an image. For example, a color image recording apparatus has been proposed which uses several sets of inks, each set consisting of inks belonging to the same color family and having different concentrations, such as light black ink and dark black ink, light cyan ink and dark cyan ink, light magenta ink and dark magenta ink, and light yellow ink and dark yellow ink, thereby improving graininess at highlighted portions. Using this head unit makes it possible to ameliorate the problem in that dots appear unnaturally in highlighted portions in an image recorded by a binary recording type color image recording apparatus.
FIG. 4 shows the schematic arrangement of a conventional color ink jet recording apparatus of this type. This recording apparatus is different from the recording apparatus of FIG. 1 in the construction of the recording head unit 9. The recording head unit 9 comprises eight recording heads 9-1BK for light black, 9-2BK for dark black, 9-1C for light cyan, 9-2C for dark cyan, 9-1M for light magenta, 9-2M for dark magenta, 9-1Y for light yellow, and 9-2Y for dark yellow, and four sets of color ink reservoirs, each of which comprises two inks of the same color family with different concentrations. The recording medium 5 is intermittently incremented by the amount of a recording by the recording head unit 9, and the recording is carried out while the recording medium 5 is stationary. More specifically, the recording head unit 9 is scanned in the direction of the arrow P along the guide rails 6 and 7 discharging ink droplets in accordance with an image signal, thereby forming a color image consisting of dot matrices on the recording medium 5.
Generally, the signal processing system of the color image recording apparatus of this type is arranged as shown in FIG. 5. Since the signal processing system has a similar construction for each of the fur color families of black, cyan, magenta and yellow, the system for one color family is described. In FIG. 5, an input image signal S0 is divided in accordance with its intensity by a dark/light separating circuit 11. More specifically, the input image signal S0 is divided into a light black image signal S1 and a dark black image signal S2 by a dark/light separating table having input and output characteristics as illustrated in FIG. 6, and the image signals S1 and S2 are sent to a multiple-to-binary converter 12 which converts the multi-level image signals S1 and S2 into binary signals, and sends them to corresponding head drivers 13-1 and 13-2. The head drivers drive corresponding heads 9-1 and 9-2, thus recording an image. Here, the head driver 13-1 and the recording head 9-1 are for light ink, and the head driver 13-2 and the recording head 9-2 are for dark ink.
FIG. 6 illustrates the driving duty of the dark and light heads for the input image signal S0. When the input image signal S0 is smaller than P (P=127 for an 8-bit signal, for example), only the light head 9-1 is used. On the other hand, when the input image signal S0 is greater than P, the recording duty of the light head 9-1 is gradually reduced whereas the recording duty of the dark head 9-2 is gradually increased. For example, consider 2.times.2 superpixels including four dots each. One superpixel can be formed by discharging ink drops up to twice from two nozzles of the dark head and the corresponding two nozzles of the light head, the nozzles being arranged in the vertical direction of each head, for instance, let us assume that, at the first discharge, only light ink droplets are discharged from the two nozzles of the light head, and at the second discharge, a light ink droplet is discharged from the upper nozzle of the light head and a dark ink droplet is discharged from the lower nozzle of the dark head. Thus, the duty of the light head is 75% and that of the dark head is 25% in forming this superpixel.
By driving two recording heads in accordance with the image signal as described above, the concentration of images can be controlled as shown in FIG. 7. Although the recorded image density characteristics as shown in FIG. 7 can be achieved by using only the dark head 9-2, using both the dark and light heads is advantageous in that the rough feeling of dots at low density portions is reduced, and a smooth gradation image can be achieved.
However, the recording by using the dark and light heads cannot sometimes accomplish sufficient effects even if the image signal correction for the edge nozzles is performed as stated before. More specifically, when the image signal correction is carried out by using the dark/light separation table whose characteristics are shown in FIG. 6, the total ink volume (the sum of the light ink volume and the dark ink volume) discharged form the heads reaches the fixed maximum after the input image signal S0 exceeds the value P as shown in FIG. 8. In the region where the image signal S0 is greater than P, both the dark and light heads are used as shown in FIG. 6. In this case, the ink volume discharged from the light head decreases as the ink volume discharged from the dark head increases, and hence, the total discharged ink volume is maintained at the fixed value. Consequently, if the maximum total ink volume is discharged from the edge discharging nozzles of the dark and light recording heads, the problem of ink running and the black lines may occur. To avoid this, the total ink volume discharged from the edge nozzles must be restricted if the input image signal exceeds a predetermined value Q smaller than P.
To achieve this, a table conversion as shown in FIG. 9 must be performed on the image signal S0 inputted to the edge nozzles. Here, the value Q is determined experimentally by practically printing an image on a recording medium so as to obtain optimum conditions, and the value S in FIG. 9 corresponds to the level Q of the input image signal S0 as shown in FIGS. 6. This conversion, however, is sometimes ineffective to restrict the ink discharge volume for the following reason. As will be seen from FIG. 6, when the image signal S0 is greater than Q, the image signals S1 and S2 applied to the dark and light recording heads will exceed the value S in the regions A and C of FIG. 6, respectively. Accordingly, the image signals S1 and S2 are restricted as shown in FIG. 9 in the regions A and C. On the other hand, the image signals S1 and S2 do not exceed S in the region B of FIG. 6 even if the input image signal S0 is greater than P. Thus, the image signals S1 and S2 are not restricted in the region B. AS a result, the total ink volume is not restricted, and the black lines are likely to occur.
The characteristic lines of FIG. 6 will now be explained in more detail. When the image signal S0 takes a value from 0 to 127 in the 8-bit representation, only the image signal S1 applied to the recording head 9-1 for light ink is outputted. This means that the optical density is achieved by recording a number of light ink dots on the recording medium so that the degradation of an image due to distinct individual dots can be prevented.
On the other hand, when the image signal S0 ranges from 128 to 255 in the 8-bit representation, the value of the image signal S2 applied to the recording head 9-2 for the dark ink after the separation by the table increases linearly with the image signal S0, thus increasing the discharge volume of the dark ink. In contrast, the image signal S1 applied to the recording head 9-1 for the light ink decreases linearly with the image signal S0 so as to restrict the total ink discharge volume. If the discharge volume of the light ink is not decreased, the total ink discharge volume will exceed the allowable ink volume of a recording medium when a plurality of color inks are superimposed, thereby causing the overflow of the ink.
To avoid the overflow of ink, UCR (under color removal) processing and inking processing of black ink are carried out so that the total ink discharge volume is reduced. More specifically, at portions where cyan, magenta and yellow inks are superimposed to represent black, the volumes of these inks are reduced, and black ink is used in place of these inks, thus reducing the total ink discharge volume. The conventional method, however, employs the image signal S0 before the dark/light separation to perform the UCR and inking processings. This presents a problem in that the total ink volume cannot be effectively reduced for the following reasons. FIG. 10 illustrates the relationship between the input image signal S0 and the recorded ink volume Vink per color (the sum of light ink and dark ink) when the image signal S0 is separated by the table as shown in FIG. 6. As will be seen from FIG. 10, the recorded ink volume of each color is maintained at a fixed value even if the image signal S0 decreases in the range from 128 to 255 in the 8-bit representation. Thus, the reduction of the recorded ink volume cannot be expected by the UCR processing. As a result, there still remains a problem that the total recorded ink volume cannot be effectively reduced.
Moreover, when the three inks of cyan, magenta and yellow (that is, the light cyan and dark cyan, the light magenta and dark magenta, and the light yellow and dark yellow) are superimposed, the total ink volume may exceed the allowable ink volume of a common recording medium, thereby causing the overflow of inks on the recording medium, even if the image signal S1 is decreased as the image signal S0 increases in the range of 128-256 in the 8-bit representation.